Tube bundle equipment such as shell and tube heat exchangers and similar items of fluid handling devices such as flow dampers and flow straighteners utilize tubes organized in bundles to conduct the fluids through the equipment. In such tube bundles, there is typically fluid flow both through the inside of the tubes and across the outside of the tubes. The configuration of the tubes in the bundle is set by the tubesheets into which the tubes are set. One common configuration for the tubes is the rectangular or square formation with the tubes set in aligned rows with tube lanes (the straight paths between the tubes) between each pair or rows, aligned orthogonally to one another. In this formation, each tube is adjacent to eight other tubes except at the periphery of the tube bundle and is directly opposite a corresponding tube across the tube lane separating its row from the two adjacent rows. In the triangular tube formation, the tubes in alternate rows are aligned with one another so that each tube is adjacent to six other tubes (the two adjacent tubes in the same row and four tubes in the two adjacent rows).
Increases in throughput in existing exchangers are often desired either to reduce capital cost by reducing equipment size or to increase productivity factors. A common limiting factor experienced when evaluating the increase of rates in an exchanger is the potential for flow-induced vibration damage of the tubes. Fluid flow patterns around the tubes may give rise to flow-induced vibrations of an organized or random oscillatory nature in the tube bundle and in the case of devices such as heat exchangers in which heat transfer takes place between the tubes and the surrounding fluid, the changes in the temperature and density of the fluid as it circulates and flows around the tubes may increase the likelihood of vibration. If these vibrations reach certain critical amplitudes, damage to the bundle may result. Tube vibration problems may be exacerbated if heat exchange equipment is retubed with tubes of a different material to the original tubes, for example, if relatively stiff materials are replaced with lighter weight tubes. Flow-induced vibration may also occur when equipment is put to more severe operating demands, for example, when other existing equipment is upgraded and a previously satisfactory heat exchanger, under new conditions, becomes subject to flow-induced vibrations. Vibration may even be encountered under certain conditions when a heat exchanger is still in the flow stream but without heat transfer taking place as well as in other tube bundle devices with collections of rods or rod-like elements in a flow stream with or without heat transfer.
A number of different equipment designs have evolved to deal with the problem of tube vibration. One example is the rod baffle design. Rod baffle heat exchangers are shell and tube type heat exchangers utilizing rod baffles to support the tubes and secure them against vibrations. Additionally, rod baffles can be used to reduce shell-side flow maldistributions and to create a more uniform shell-side flow. The term “baffle” refers to the annular rings, placed every 15 cm or so along the length of the tube bundle, in which the ends of a plurality of support rods are connected to form a cage-like tube support structure; hence the term “rod baffle”. Rod baffle exchangers, however, tend to be approximately 30 to 40% more expensive than conventional shell-and-tube exchangers and there have been situations where tube bundle devices of this kind have failed owing to flow-induced vibrations. Rod baffle heat exchanges are described, for example, in U.S. Pat. Nos. 4,342,360; 5,388,638; 5,553,665; 5,642,778.
As explained in U.S. Pat. No. 5,553,665, certain applications of the rod baffle design such as surface condensers and power plant applications may benefit from longitudinal-flow, with shell-side pressure losses to be minimized. Reduction in shell-side pressure losses may be accomplished by increasing rod baffle spacing, thereby reducing the number of rod baffles, or by decreasing the number of tubes by increasing the tube pitch dimension, i.e., the distance between two adjacent rows of tubes as measured from the center of the tubes. Increasing baffle spacing is usually not an attractive option, since increased baffle spacing increases the likelihood of flow-induced tube vibration occurrence. Decreasing the tube count by increasing tube pitch dimension produces decreased shell-side pressure loss for longitudinal-flow between rod baffles, but requires oversized support rod diameters, leading to increased rod baffle pressure losses, which may offset any decrease in longitudinal-flow, shell-side pressure loss resulting from the reduced tube count. This would also lead to a more expensive exchanger owing to the increased shell diameter for a specified tube count. The rod baffle design described in U.S. Pat. No. 5,553,665 represents an attempt to deal with the pressure drop problems of the rod baffle configuration.
An alternative design is the “Eggcrate” design. This, however, is even more expensive than the rod baffle design while it also allows tube chatter that could lead to tube failure. Chatter is the motion of a tube hitting the tube supports because of the gap between the support and the tube outside diameter. The gap is required to allow for inserting the tubes through the eggcrate support when the bundle is being constructed. From the economic and operational viewpoints, therefore, the road baffle design represents a more hopeful starting point.
Besides good equipment design, other measures may also be taken to reduce tube vibration. Tube support devices or tube stakes as these support devices are commonly known (and referred to in this specification) may be installed in the tube bundle in order to control flow-induced vibration and to prevent excessive movement of the tubes. A number of tube supports or tube stakes have been proposed and are commercially available. U.S. Pat. No. 4,648,442 (Williams), U.S. Pat. No. 4,919,199 (Hahn), U.S. Pat. No. 5,213,155 (Hahn) and U.S. Pat. No. 6,401,803 (Hahn), for example, describe different types of tube stake or tube support which can be inserted into the tube bundle to reduce vibration. Improved tube stakes are shown in U.S. patent application Ser. No. 10/848,903, filed 24 Jun. 2003, entitled “Anti-Vibration Tube Support” of A. S. Wanni, M. M. Calanog, T. M. Rudy, and R. C. Tomotaki.
We have now devised a tube bundle device, for example, a heat exchanger which is believed to be more effective, more reliable, more easy to fabricate and less expensive than a conventional heat exchanger of the rod baffle type. According to the present invention, a tube support cage (TSC) similar to a rod baffle is placed at extended locations along the length of the tubes, e.g. every 60–100 cm apart, thereby making fabrication of such a tube bundle much easier and less expensive, as compared to conventional rod-baffle devices, in which the rod-baffle supports are typically placed no more than approximately 15 cm apart. The tube bundle is stiffened by inserting tube stakes between the tube support cages, preferably at the midpoint of the tube span between the cages. The preferred type of tube stake is the type described in copending U.S. patent application Ser. No. 10/848,903, referred to above but other stakes might also be used.
According to the present invention, the tubes are supported by rods or flat bars in each tube lane at the TSC locations, compared to the cages provided in every other tube lane in the rod baffle design. The rod baffle design requires four distinct types of baffles with support rods in alternate tube lanes at alternate axial locations, both horizontally and vertically, but the current invention is simpler requiring only two types: one with horizontal rods (or flat bars) and the other with vertical rods (or flat bars). As another advantage, the current invention prevents or reduces the tube chatter resulting from insufficient tube support as well as the possibility of flow-induced vibrations exacerbated by the chatter. Chatter is often considered to be essentially unavoidable in rod baffle type exchangers unless the rod diameter is very closely equal to the spacing between the adjacent tubes. However, the smaller the gap between the rods and the adjacent tubes, the more difficult and costly is the assembly of the bundle.
In general terms, the tube bundle device according to the present invention uses tube support cages which alternate with the sets of tube support stakes axially along the tubes. The orientation of the tube support members of each cage is rotated axially with respect to the tube support members of each axially adjacent cage and the orientation of the tube support stakes of each set is rotated about the axis with respect to the tube support stakes of each axially adjacent set. A number of different types of tube support stake may be used, preferably of the type which will deviate the tubes slightly to engage with the support members of the cages, comprising longitudinally extensive strips with successive transverse rows of raised, tube-engaging zones on each face of the strip which extend laterally outwards from both faces of the strip to engage with the tubes on the opposite sides of the tube lane into which the stake is inserted.
The invention is primarily applicable to the rectangular tube arrangement but could be applied also to the triangular configuration with the axial rotation of the successive cages and sets of stakes being in accordance with the type of arrangement. In rectangular arrangements, the support members in the cages will be rotated 90° from horizontal to vertical to horizontal successively and similarly for the alignment of the successive sets of stakes. In the triangular tube arrangement the rotation will be 60° or 120° at successive locations. The use of the triangular arrangement allows fabrication of less expensive exchangers by decreasing the shell diameter for a specified number of tubes although one drawback is that the support structure is not as strong as for the inline arrangement.
In this specification and claims, the terms “vertical” and “horizontal” are used in the relative sense with respect to the orientations of the elements of the tube support cages and of the stakes, that is, to designate a relative orientation of the support cage elements or of the stakes with respect to one another and the axis of the device. Thus, references to the “vertical” orientation mean that the orientation is orthogonal to a specified “horizontal” orientation, without implying that the orientations are true vertical or true horizontal. This applies especially when the axis of the heat exchanger itself is vertical or horizontal, so that all the support cages and stakes will be at true horizontal. Thus, the references to “vertical” and “horizontal” in relation to the orientation of the elements of the tube support cages and of the stakes are to be taken on the assumption that the longitudinal axis of the tube bundle device is itself true horizontal and that the specified orientations are relative to one another not true. For example, in a heat exchanger with a true horizontal longitudinal axis, the elements of the tube support cages may be at angles of 45° to the true horizontal/vertical but still be “vertical” and “horizontal” with respect to each other. In a heat exchanger with a vertical longitudinal axis, all the elements of all the tube support cages will be at true horizontal but are nevertheless to be considered to be “vertical” and “horizontal” if their orientations relative to one another about the longitudinal axis are orthogonal.
Normally, the cages will alternate along the length of the tubes with the sets of tube stakes: Stake Set 1, Cage 1, Stake Set 2, Cage 2, Stake Set 3, Cage 3, Stake Set 4 and so on. The orientations of the cages will be rotated about the longitudinal axis at successive axial locations so that Cage 2 is rotated with respect to Cage 1 and Cage 3 with respect to Cage 2; in the rectangular arrangement with two successive 90° rotations, Cage 3 will revert to the same alignment as Cage 1. In the triangular tube arrangement, a rotation of a multiple of 60° (i.e. 60° or 120° with further successive rotations restoring previous alignments) is made at each axial location. Similarly, the orientations of the stake sets will preferably be rotated about the longitudinal axis at successive axial locations with the stakes inserted parallel to the support rods of an adjacent support cage next along the axis of the bundle. So, in a rectangular tube arrangement, Stake Set 2 is rotated 90° with respect to Stake Set 1 and Stake Set 3 with respect to Stake Set 2 and with two successive 90° rotations, Stake Set 3 will revert to the same alignment as Stake Set 1. In the triangular tube arrangement, three successive 60° rotations will restore the original alignment.
In the fabrication of the tube bundle, the tubes are inserted through each tube support cage and into one or both tubesheets to form the bundle with defined tube lanes between adjacent rows of tubes. At this point, there is, desirably, some clearance between the tubes and the support members as the support members of each cage are spaced apart from one another to allow the clearance or play between the tubes and the support members. Because the tubes fit into the tube support cages with some clearance, it is possible to insert the tubes more readily than with the conventional tight fitting rod and baffle design. The tube stakes are then inserted into the defined tube lanes at each location along the tubes from the cage(s). The stakes are inserted so that they are aligned parallel to the tube support members of an axially adjacent support cage to impart an increased separation between the tubes so that they are urged against the support members of the adjacent cage to take up the clearance and hold the each tube against one or another support member. In this way, fabrication of the tube bundle is facilitated while a final, rigid, vibration-resistant tube bundle is achieved.